The overall objective of the proposed research is to develop non-invasive optical devices for the detection and diagnosis of early dental caries (dental decay). New, more sophisticated diagnostic tools are needed for the detection and characterization of caries lesions in the early stages of development. If carious lesions are detected early enough, they can be arrested/reversed by non-surgical means through fluoride therapy, anti-bacterial therapy, dietary changes, or by low intensity laser irradiation. The principal factor limiting optical imaging through the enamel of the tooth in the visible range of 400-700-nm is light scattering. Light scattering in sound enamel and dentin is sufficiently strong in the visible range to obscure light transmission through the tooth. The magnitude of light scattering in dental enamel decreases markedly with increasing wavelength. Therefore, we hypothesize that the near-IR region from 830-1550-nm offers the greatest potential for new optical imaging modalities due to the weak scattering and absorption in sound dental hard tissue. At longer wavelengths, absorption of water in the tissue increases markedly reducing the penetration of IR light. The overall objectives of this proposal will be achieved through the following specific aims: (1) Measure the optical constants and light scattering anisotropy and phase function of sound dental hard tissue at wavelengths in the near- IR between 660 and 1550-nm for polarized and unpolarized light and determine the changes in those optical parameters that occur upon demineralization during the caries process; (2) Develop near-IR polarization sensitive optical coherence tomography (PS- OCT) for the detection, diagnosis, and imaging of early caries lesions and for the monitoring of lesion progression in simulated caries models; (3) Develop near-IR transillumination for the detection and imaging of early interproximal caries lesions. It is likely that if these studies and future clinical trials are a success, that this novel technology for imaging dental hard tissue will be employed for the detection and monitoring of early carious lesions without the use of ionizing radiation, thereby enabling conservative non-surgical intervention and the preservation of healthy tissue structure. Moreover, it is probable that this proposed imaging technology would enable the clinician to detect and quantify the severity of occlusal lesions that are not resolvable with conventional radiography due to the surrounding sound tissue structure.